Nanoparticles are ultrafine particles whose size is below 1 μm, often less than 100 nm. They have received considerable attention because of their unique properties afforded by their small size and high surface-to-volume ratio. Nanoparticles are utilized in various technical fields, spanning from semiconductors, nanocomposite materials, catalysis and biomedical applications.
There are numerous possibilities for producing nanoparticles, such as sol-gel, chemical vapor deposition (CVD), and hydrothermal and spray pyrolysis methods. In recent years, methods for producing particles using plasma have been developed. These processes can be used for nucleation of particles, growth of particles and deposition of coatings on particle surfaces. Processes utilizing plasma inter alia have the advantages of small amounts of by-products, high purity of particles formed, and easy separation of particles from carrier gases.
One example of such a method is disclosed in US 2008/0271987 wherein nanoparticles are formed by introducing a process gas and an ambient gas into a reaction chamber and applying a plasma in a pulsed mode. Other examples of methods for producing nanoparticles by means of plasma are disclosed in for example U.S. Pat. No. 7,297,619 and US 2009/0014423.
A particular example of methods using plasma for producing particles, especially nanoparticles, is the plasma sputtering process. Sputtering is the physical ejection of atoms from a target surface. In the process, ions, for example argon ions, are generated in the plasma and drawn out of the plasma and accelerated across a cathode dark space. The target has a lower potential than the region in which the plasma is formed and the target surface therefore attracts positive ions. The positive ions move towards the target with a high velocity and then impact the target and cause atoms to physically dislodge or sputter from the target surface.
The plasma sputtering process inter alia has the advantage that the particles are formed from material of a solid target. The target can easily be manufactured and used in the process. A solid target is especially suitable in case particles are to be formed from metallic materials since it eliminates the need for metal species containing gases which often are toxic and require complex manufacturing methods.
DE 198 04 838 discloses a plasma sputtering process for depositing coating material or producing radicals on particle surfaces using a hollow cathode system. The process is conducted at a pressure of 0.01 mbar to atmospheric pressure, preferably 0.01-100 mbar. The plasma may be a pulsed plasma, which is achieved by controlling the pressure and operated at a power frequency of 50-60 Hz. A voltage of 200-500 V and a current of 0.1-2 A is used to form the plasma.
The problem with previously known plasma sputtering processes for production of particles is that the pick-up probability of atoms on the surface of particles generally is low. Therefore, the particles and/or the coatings have a slow growth rate, thus leading to low production rates of particles. Furthermore, since the production rate is so low, these methods are less suitable for production of particles which have a size in the range of a few micrometers.